In recent years, image forming apparatuses employing electrophotography and an ink jet recording system have come to be used in the field of on-demand printing. Yet even users in the field of the quick printing (on-demand printing) demand high quality production from such image forming apparatuses. To meet that demand, manufacturers adjust product quality, in particular color reproduction concentration, at shipment.
In addition, image quality is also affected by the course of time, environmental changes, changes in characteristics of developing agents, etc., due to usage conditions thereof, and usage of paper having different characteristics. Consequently, users also must adjust color reproduction concentration to maintain the quality of images for such changes over time, environmental changes, and characteristic changes.
Image forming apparatuses employing electrophotography convert image data formed of the colors red (R), green (G), and blue (B) image data or cyan (C), magenta (M), yellow (Y), and black (K) into reproducible colors according to color reproduction models in the apparatuses. Images are formed on recording media using image forming materials such as toner and ink constituted of, for example, the colors yellow, magenta, cyan, and black. The color conversion described above is conducted by using color conversion profiles stored in the image forming apparatuses. The color conversion profiles are made based on a color reproduction model at the stages of designing, manufacturing, and shipment. That color reproduction model indicates the range of colors reproducible by an image forming apparatus and is expressed as a characteristic diagram of three axes of X, Y, and Z, mapping intensity, saturation, and color phase, respectively. A simpler color reproduction model can be provided in which colors are expressed using saturation for the X axis and intensity for the Y axis.
A color chart is another color reproduction model in which color patches representing colors reproducible by an image forming apparatus are arranged. The color chart is output by an image forming apparatus and measured by a colorimeter to make the color conversion profile described above.
Naturally, the range of reproducible colors of an image forming apparatus subjected to color reproduction concentration adjustment depends on the color chart serving as the color reproduction model.
The reproduction colors of the color patches forming the color chart are expressed as predetermined pile heights. The pile height is the total of the physical amount of an image forming material formed of, for example, yellow, cyan, magenta, and black in proportion to the color reproduction concentration of the four colors.
Therefore, the reproducible colors of the color patch are expressed as the total of the physical amount in proportion to the color reproduction concentration of the four colors.
FIG. 5 is a diagram illustrating one of multiple color blocks forming a typically used color chart. A color block 10 illustrated in FIG. 5(a) includes color patches 11 arranged in a matrix (of 12×12 patches in this example) which have different color reproduction concentrations formed by increasing the color reproduction concentrations of cyan (C) and magenta (M) (from among cyan (C), magenta (M), yellow (Y), and black (K)) at a fixed rate.
In FIG. 5(a), the ratio of the color reproduction concentration of each of yellow (Y) and black (K) is 0%.
While the color reproduction concentration of one of the two colors selected from the four colors is kept constant, color patches are formed by increasing the color reproduction concentration of the other color of the selected two at a fixed rate and arranged vertically or the row direction.
For example, the color block 10 illustrated in FIG. 5A is constituted according to the following layout procedure of multiple color patches 11. A color patch column group 12 is formed by sequentially laying out multiple color patches 11 formed by increasing the color reproduction concentration of magenta (M) vertically at a fixed rate while the color reproduction concentration of cyan (C) is kept unchanged at a predetermined ratio.
The color patch column group 12 is copied horizontally while increasing the color reproduction concentration of cyan (C) at a predetermined fixed rate. This is repeated while increasing the level of the predetermined fixed rate one by one until the thus-formed multiple copied color patch column groups with different levels of color reproduction concentration are laid out horizontally (from left to right in FIG. 5(a)) according to the color reproduction concentration to obtain the color block 10.
A color patch row group 13 is formed by sequentially laying out multiple color patches 11 formed by increasing the color reproduction concentration of cyan (C) level by level at a fixed rate while the color reproduction concentration of cyan (M) is kept at a predetermined color reproduction concentration unchanged. The color patch row group 13 is copied vertically while increasing the color reproduction concentration of cyan (M) one level of a predetermined fixed rate. This is repeated while increasing the level of the predetermined fixed rate one by one until the thus-formed multiple copied color patch row groups with different levels of the fixed rate of color reproduction concentration are laid out vertically (from top to bottom in FIG. 5(a)) according to the color reproduction concentration to obtain the color block 10.
To the color block 10 of FIG. 5(a), in which the ratio of the color reproduction concentration of yellow (Y) and black (K) to the total of the color reproduction concentration of the four colors is zero, the ratio of the color reproduction concentration of yellow (Y) is raised one level of a predetermined fixed rate for all the color patches 11 in the color block 10 to obtain a color block 20 illustrated in FIG. 5(b). The ratio of color reproduction concentration of black (K) to the color reproduction concentration of the colors is still kept at 0%. Then, other color blocks having different color reproduction concentrations of yellow (Y) are repeatedly formed as described above until all the color blocks laid out as in FIG. 6 (four color blocks 10, 20, 30, and 40 in FIG. 6). Thereafter, the color blocks 10, 20, 30, and 40 illustrated in FIG. 6 are output on the surface of recording paper serving as an output medium to form a color chart 50. To be specific, the color block 20 is laid out adjacent to the color block 10 horizontally.
The color block 30 is laid out not adjacent to the color block 20 horizontally but to the color block 10 vertically if it cannot be output on the surface of the recording paper because of the size of the surface as illustrated in FIG. 6. Furthermore, the color block 40 is laid out adjacent to the color block 30 horizontally. As a result, it can be seen that, in the color chart 50 having such an arrangement, for example, the concentration difference between a color patch 51-1 and a color patch 51-2 laid out adjacent to the color patch 51-1 horizontally is extremely large, as illustrated in FIG. 6. The implications of this large difference are explained below.
Next, the color reproduction concentration in the image forming apparatus using the color chart 50 output must be adjusted. First, a color patch, for example, the color patch 51-1 in the reading spot indicated by broken lines in FIG. 6, is read by a colorimeter, which is a device that irradiates a reading target with light and receives only reflected light therefrom to output image data. Then, a higher-level device temporarily stores the read image data and compares them with reference data of the color patch present corresponding to the arrangement in the color chart preliminarily stored in a memory table. The color reproduction concentration in the target image forming apparatus is adjusted based on this comparison result.
However, as described above, when image data expressed as RGB color space is color-converted into and expressed as YMCK on the output side, the combination of RGB is 256×256×256, which is about 16,700,000 if one color has 256 gradations. Such a large combination of colors requires a number of sheets to obtain a printed color chart shown in FIG. 6.
In such a case, reading each of color patches by a colorimeter while focusing beams of light thereon takes an extremely long time to complete reading. To make the color reading time short, it is conceivable to reduce the number of the gradations and combinations. However, the color reproduction concentration distribution of each arranged color patch is rough, which results in a large adjustment error of the color reproduction concentration. Therefore, drastically reducing the number of gradations and combinations is not desirable.
In an attempt to shorten the time taken for color measuring, Japanese patent application publication no. 2001-088357 (JP-2001-088357-A) describes a method of reading multiple color patches of a color chart in a two-dimensional range. However, in the method of JP-2001-088357-A described above, to obtain the color patch in the color chart in the two-dimensional range, the image data of the color patch serving as the measuring target are affected by adjacent color patches. To be specific, as illustrated in FIG. 8(a), among YMCK of a target color patch 70 in part of the color chart, if the color reproduction concentration of, for example, cyan (C) of the target color patch 70 is much lower than those of the color patches 71 to 74 laid out adjacent to the target color patch 70, those color patches are read by a two-dimension scanning-type image pickup. In addition, as illustrated in FIG. 8(b), even when the color reproduction concentrations of cyan (C) of the color patches 71 to 74 are zero, those color patches are read by the two-dimension scanning-type image pickup.
These drawbacks stem from the scanning method of a scanner in which the reflected light of the color patches and paper around a target color patch is read as well as the reflected light from the target color patch irradiated with light in the two-dimensional range. Therefore, as illustrated in FIG. 8(c), by making the color reproduction concentration of, for example, cyan (C) among YMCK of the target color patch 70 same as those of the color patches 71 to 74 around the target color patch 70, the concentration components in the measuring data are the same or significantly close to each other if the reflected light from the color patches 71 to 74 in the two-dimensional range is taken in. Therefore, the image data of the target color patch 70 are much less affected.
However, the area of the same color significantly increases by laying out color patches having the same color around the target color patch, which in turn greatly decreases the number of target color patches in the color chart output on paper.